Japan opened mass inoculation centres on Monday as the country races to vaccinate most of its elderly population against COVID-19 before the start of the Tokyo Olympics.

The centres in Tokyo and Osaka will vaccinate thousands of people every day, giving a boost to Japan's sluggish inoculation drive as officials battle a fourth wave of infections.

"It's better to get it early," said Tetsuya Urano, 66, who was among the first to be vaccinated in Tokyo. "It went pretty smoothly, all in all."

The Tokyo facility will operate 12 hours a day to dispense shots to some 10,000 people daily for the next three months. The site in Osaka, Japan's western metropolis, will build up to about 5,000 shots a day.

Prime Minister Yoshihide Suga called for the centres last month to speed up the country's vaccination rollout. Large-scale inoculation sites operated by local governments also opened in the prefectures of Aichi, Miyagi, and Gunma.

The fourth wave of infections has led authorities to make state of emergency declarations covering much of the country, including Tokyo, raising some concerns about the Olympic Games due to begin on July 23.

The states of emergency for most regions are due to end on May 31, but the government is planning to extend them to June 20, the Yomiuri newspaper reported.

Just 4.4% of Japan's population of 125 million have received at least one vaccine dose, according to Reuters' global tracker, the slowest rate among the world's larger, rich countries.

Japan began its inoculation push in mid-February, later than most major economies. The campaign was slowed initially by scant supplies of imported doses of the vaccine developed by Pfizer Inc (PFE.N) and BioNTech SE (22UAy.DE). But even as shipments increased, the rollout has been hampered by manpower shortages and malfunctions in the reservation system

The mass vaccination centres for the elderly are using Moderna Inc's (MRNA.O) vaccine, which was approved on Friday, along with AstraZeneca PLC's (AZN.L) vaccine.

On Monday, Johnson & Johnson (JNJ.N) said it had filed for regulatory approval of its one-shot candidate and it could begin supplying the country in early 2022.